(a) Field of the Invention
The present invention relates to a method for washing wafers and an apparatus for washing wafers, more in particular to the method and the apparatus for washing the wafers in which the removing rate of particles deposited onto the wafers is higher than that of a conventional method or apparatus.
(b) Description of the Related Art
In the manufacture of a semiconductor device, a wafer is washed for removing foreign substances, impurities and contaminations upon completion of each of process steps. The wafer is also frequently washed in pretreatment steps such as an initial washing step before processing a silicon substrate, a pre-oxidation washing step before deposition of a silicon oxide film on the silicon substrate, a washing step before ion implantation and a further washing step before deposition of a CVD film
As shown in FIGS. 1 and 2, a conventional wafer washing apparatus 10 known as a single vessel-type apparatus includes an inner chamber 12 for washing wafers with a washing solution composed of pure water and a chemical accommodated therein, an outer chamber 14 which surrounds the periphery of the inner chamber 12 and to which the washing solution overflows from the inner chamber 12, a pure water supplying apparatus (not shown) and a chemical solution supplying apparatus (not shown).
In the wafer washing apparatus 10, a plurality of wafers xe2x80x9cWxe2x80x9d disposed on a lifter 16 in an upright posture and separated from one another are transferred into the inner chamber 12 and washed at a time.
The lifter 16 includes a mounting table 16a having, for example, a width xe2x80x9cW1xe2x80x9d of 140 mm and a depth xe2x80x9cL1xe2x80x9d of about 180 mm and a gripping plate 16b extending upright from the deepest edge of the mounting table 16a. 
The mounting table 16a has a central bottom line and is higher toward the both ends of the table. Both the ends are higher than the central bottom line by a height xe2x80x9cH1xe2x80x9d of 30 mm. The mounting table 16a includes a plurality of holding trenches 16c extending parallel to the gripping plate 16b and separated from one another. The wafer xe2x80x9cWxe2x80x9d is fixed onto the mounting table 16a by engaging the bottom of the wafer xe2x80x9cWxe2x80x9d into the holding trench 16c. 
The inner chamber 12 is rectangular parallelepiped with a top opening having dimensions capable of accommodating the lifter 16 with the plurality of wafers, for example, a width xe2x80x9cW2xe2x80x9d of 220 mm, a depth xe2x80x9cL2xe2x80x9d of 200 mm and a height xe2x80x9cH2xe2x80x9d of 230 mm, and an inner volume thereof is about 10 liters.
The inner chamber 12 further includes pipes 18A and 18B having nozzles for ejecting the chemical solution and the pure water at both ends of the width direction of the inner chambers 12, and a QDR (quick dump rinsing) drain port 20 having a large opening at the bottom thereof for rapidly discharging the chemical solution and the pure water in the inner chambers 12.
The outer chamber 14 includes sidewalls disposed outer than those of the inner chambers 12 by about 30 mm, designated by xe2x80x9cSxe2x80x9d in FIG. 1. The sidewalls of the outer chamber 12 have a height xe2x80x9cH3xe2x80x9d of 230 mm which is lower than the sidewalls of the inner chamber 12 by 0 to 5 mm (xcex94h) as shown in FIG. 1. The outer chamber 14 further includes, at the bottom thereof, a port 22 for discharging the pure water and d the chemical solution overflown from the inner chamber 12.
Referring to a flow chart of FIG. 3 in addition to FIGS. 1 and 2, the method of washing the wafers by using the conventional wafer washing apparatus 10 will be described.
At first, pure water is supplied to the inner chamber 12 at a rate of 5 liter/min. through the pipes 18A and 18B and is allowed to be over-flown from the inner chamber 12 to the outer chamber 14. While maintaining the overflow of the pure water, the lifter 16 mounting the wafers is dipped into the pure water in the inner chamber 12 (S1: wafer dipping step).
Then, pure water is supplied to the inner chamber 12 at a flow rate of 20 liter/min. for 2 minutes through the pipes 18A and 18B (S2: pure water upward flowing step).
After the supply of the pure water is stopped, the chemical solution including ammonia (NH4OH), hydrogen peroxide (H2O2) and pure water in a volume ratio of 1:1:50 is supplied to the inner chamber 12 at a flow rate of 20.8 liter/min. for 120 seconds through the pipes 18A and 18B for washing the wafers (S3: chemical solution upward flowing step).
After the supply of the chemical solution is stopped, the so-called dip washing is conducted for about 480 seconds while the wafers disposed on the lifter 16 are dipped in the chemical solution (S4: dip washing step).
Only pure water is supplied to the inner chamber 12 at a flow rate of 20 liter/min. for 30 seconds, in view of the fact that the volume of the inner chamber 12 is about 10 liters, through the pipes 18A and 18B for replacing the chemical solution in the inner chamber 12 with the pure water (S5: pure water replacing step).
Then, QDR is conducted (S6: QDR step). In the QDR washing, at first, the QDR drain port 20 is released to discharge the pure water in the inner chamber 12 supplied thereto in the pure water replacing step S5. After the QDR drain port 20 is stopped to fill the inner chamber 12 with the pure water, the QDR drain port 20 is released again to discharge the pure water in the inner chamber 12. The procedures are repeated five or six times.
Then, only pure water is supplied to the inner chamber 12 at a flow rate of 20 liter/min. for 90 seconds through the pipes 18A and 18B, and a specific resistance of the pure water in the inner chamber 12 is confirmed to be substantially same as that of the pure water before the supply to the inner chamber 12, the lifter is lifted to transfer the wafers to a drying chamber not shown (S7: finish washing step).
When the wafers are washed in accordance with the above procedures by using the single vessel-type wafer washing apparatus, the removing rate of particles deposited onto the wafer is poor and the washing of the wafers cannot be conducted satisfactorily.
The number of the particles deposited onto the wafer after the washing is high, and the washing is insufficient as a pretreatment or a post-treatment of a process for forming a finer pattern in a semiconductor device.
With the higher miniaturization and the higher integration of the semiconductor device, the miniaturization is more and more demanded together with the higher accuracy. Accordingly, the sufficient washing of the wafers meeting the higher accuracy processing cannot be achieved in the conventional wafer washing method.
In view of the foregoing, an object of the present invention is to provide a method for washing wafers and an apparatus for washing wafers which can be satisfactorily used as a pretreatment or a post-treatment of processing a semiconductor device by employing a single vessel-type wafer washing apparatus.
Thus, the present invention provides, in a first aspect thereof, a method including the consecutive steps of: dipping a wafer in a washing solution in a washing chamber; replacing the washing solution by a first chemical solution in the washing chamber receiving therein the wafer, the first chemical solution including at least one chemical; dipping the wafer in the first chemical solution after stopping the replacing; and replacing the first chemical solution by a second chemical solution including the at least one chemical and having a concentration lower than a concentration of said first chemical solution.
In accordance with the present invention, the particles floating in the first chemical solution is over-flown from the washing chamber together with the first chemical solution by replacing the first chemical solution with the second chemical solution, thereby discharging the particles from the washing chamber. Even if the particles are re-deposited onto the wafer in the second chemical solution supply step, the deposited particles are released by the releasing ability of the second chemical solution, thereby overflowing the particles from the washing chamber together with the first chemical solution.
Accordingly, the number of the particles remaining on the wafer is significantly reduced compared with a conventional method to improve the removing rate of the particles deposited onto the wafer.
The present invention provides, in a second aspect thereof, a wafer washing apparatus including a washing chamber having drain port at a bottom thereof, a wafer mount for mounting thereon a plurality of wafers in an upright posture with a space therebetween, and a washing nozzle ejecting a washing solution to the wafers in an oblique and downward direction.
In accordance with the second aspect of the present invention, the washing apparatus for suitably conducting the washing method of the first aspect can be realized.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.